In the case of Japanese freshwater eels raised in an artificial feeding environment, gonadotropin hormone (GTH) synthesis ability of the pituitary gland is insufficient and thus the gonads do not develop (Nagahania and Yamamoto, 1973). Such glycoprotein-type GTH is essential for gonad formation and development of fish and vertebrates (Kumar et al., 1997; and Ma et al., 2004).
In this regard, it was reported that, in the case of female freshwater eels, vitellogenesis was completed by repeatedly injecting a salmon pituitary extract (SPE) as a kind of exogenous GTH into the peritoneal cavity, and 17α, 20β-dihydroxyprogesterone (DHP) induced final maturation and ovulation (Yamamoto and Yamauchi, 1974; Ohta et al., 1996; and Adachi et al., 2003), and, in the case of male freshwater eels, spermatin was induced by repeatedly injecting human chorionic gonadotropin (HCG) into blood.
Here, referring to results for artificial ovulation induction of freshwater eels, SPE injection frequency depends upon SPE injection time. Such a result may occur by different GTH reactivity for the ovary according to seasons and a reproduction rate (ovulation rate, fertilization rate and hatching rate) due to the difference.
Furthermore, when SPE is repeatedly added to broodfish of freshwater eels so as to induce artificial ovulation, an administration amount for each individual becomes excessive or deficient. As a result, quality of artificially ovulated ovaries is affected.
In addition, salmon are anadromous fish whereas freshwater eels are catadromous fish. Accordingly, salmon moves to fresh water for spawning whereas freshwater eels move to seawater for spawning, whereby salmon and freshwater eel exhibit completely different ecological characteristics. Since species specificity of fish such as freshwater eels is not greatly unique, a pituitary gland extract (SPE) from salmon is used to promote ovulation. However, there is a need for research into a method of applying a pituitary extract from flathead mullets having ecological characteristics similar to freshwater eels so as to promote ovulation and spawning under ovulation and spawning conditions set similarly to the natural environment.